P-Adrenergic antagonists (^-blockers) are effective ocular hypotensive agents that act by decreasing the formation of aqueous humor (Sugrue, 1989). Timolol, a nonselective P-adrenergic antagonist, was introduced in 1978 for the treatment of glaucoma. Today, timolol is one of the most frequently prescribed drugs for this disease. In addition to timolol, betaxolol, carteolol, levobunolol and metipranolol are currently in clinical use. Additionally, various other P-blockers, such as atenolol, labetalol, metoprolol, nadolol, pindolol, and propranolol, have been evaluated for their topical ocular hypotensive activities.
The clinical acceptance of ophthalmic P-blocker therapy, especially with a nonselective P-blocker, is usually limited by systemic side effects. For example, timolol eyedrops have caused serious cardiovascular, respiratory, and central nervous system side effects (Nelson et al., 1986). P-Blockers are only moderately lipophilic. Thus, more lipophilic prodrugs have been studied to enhance their ocular absorption. Enhanced ocular absorption would allow the use of smaller topical doses, which would decrease systemic absorption and thus the risk of systemic side effects.
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